Originally Posted by mikieg
what i saw was, there was a discussion. he provided input with references. someone did not like the input. now he is gone.
every forum has guys that know everything. usually there is some title under their avatar. if any person fractures their all knowing status, they get rid of that person.
of course this only works within a micro environment, one like a forum such as this. but out in the real world, this would not happen. i think i would put my money on the guy that had built thousands of boats over the last 60 years versus some johney come lately internet guru. but thats just my observation on what i can see from my house. prolly get banned for stating the obvious. some people are very insecure.
As my fellow moderator, Faster, noted, we do not discuss the specific details that form the basis for banning an individual member except with the member who was banned. This banning was due solely to rules violations and had nothing to do with the technical disagreements discussed above. It should be noted that all of the moderators and the site administrator reviewed the violations and unanimously agreed to the actions taken.
But I did want to discuss the technical matters on which there was a disageement and wanted to wait until all had posting priviledges restored and could respond. W.D.Schock disagreed with my characterization of fractional rigs as requiring smaller stays and shrouds, and also objected to my assertion that cored hulls are generally significantly stronger and stiffer than an equal weight non-cored hull. I thought it might be useful for me to explain why I believe my statements to be true and present this for an open discussion on these points.
REQUIRED SHROUD SIZES BEING SMALLER ON A FRACTIONAL RIG VS MASTHEAD RIG....
In terms of the fractional vs masthead rig discussion, When you actually calculate and compare the loads on rigging, you quickly become very aware that the diagonals of fractional rigs of this era (and most eras) tend to have a flatter angle than masthead rigs of that same era. To calculate the tension on a shroud, the side force load needs to be resolved through vectors into an axial force along the shroud. The steeper the angle of the diagonal, the higher the axial load relative to the horizontal load and because of that the axial tension load can easily be several times greater than the horizontal load.
So the typically flatter angles of the diagonals on a fractional rig means that the axial tension is significantly smaller relative to the side force involved. Again if the axila load is smaller on the shroud, there will be a smaller the axial compressive force imparted into the spar potentially allowing a relatively smaller cross sectional area spar section as well, and with smaller axial load, a smaller area spar section which can also tolerate more bend without buckling.
The smaller loads on fractional rig shrouds and stays also cumulatively result from a collection of other factors which include:
Actual forestay loads are proportionate to the overall the size of the headsail and are magnified by the catenary action of the forestay which in turn is impacted by the relative amount of forestay sag tolerated in the rig design.
While not so great for pointing ability, fractional rigs generally have a greater headstay sag relative to their headstay length. That greater sag (deeper catenary) means a smaller tension stress on the stay relative to the overal side force.
In addition to the load reduction due to proportionally greater sag in a fractional rig's headstay relative to a masthead rig, fractional rigs generally have smaller headsail area.
Since there is a substantially smaller forestay tensional load, the backstay has a smaller force to resist. But there is also a relatively large reduction in backstay loads on a fractional due to the leverage provided by the portion of the mast above the hounds. The larger mainsail, and threfore higher mainsheet forces also help reduce the force that the backstay is required to exert.
The cantilevering of the spar end above the hounds on a fractional rig adds one extra panel to the spar, and the ability to taper the end of a fractional rig spar more agressively due to smaller axial loads, allows the force distribution to more evenly match the section properties of the spar and greatly reduces the combined bending/axial unit
load in the spar.
While not included in the shroud load calculations, the higher axial loads in a masthead rig, mean that less mast deflection can be tolerated and therefore, indirectly there is a tendancy to design masthead rig shrouds for less elongation than is tolerated in a fractional rig, which encourages the use of rod rigging for a similar sized load.
While it may be argued that the greater flexure in a typical fractional rig vs masthead, (especially during the period of the S30 and J30) is not all that great for pointing ability, it does have an impact on the rigging size requirement and contributes to the reason that masthead rigs need large shrouds.
Because it is more critical to control flexure in a masthead rig, shroud elongation becomes more of an important design criteria. The amount of elongation is determined by shroud length, and the proportion of load to the load capacity of the shroud material. Since masthead rigs have proportionately longer shrouds, have higher tension on their shrouds (as explained above), and tolerate less elongation, their shroud sizes on a masthead rig need to be larger. And all of this is the basis for my point about the rigging sizes needing to be larger on a masthead rig.
If you think I have any of this wrong, feel free to comment. I have placed it here in order to hear how and why.
STRENGTH OF CORED VS NON-CORED PANELS:
All other things being equal, for any given weight a cored panel will have significantly more bending strength and greater stiffness, and depending on the core material, will have similar if not greater sheer strength. The basic concept in this is that a cored hull behaves like a 'I' beam, with the skins acting as the flanges and the core acting as the web. In engineering terms the greater depth of the panel increases the section modulus and moment of inertia by increasing the depth between the axially loaded flanges of the member.
The only area where the question of relative strength of cored vs uncored sections is controversial is in puncture. There is not one universally right answer here because there are so many variables between how any given cored hull vs any give non-cored hull may be constructed, and these variables have a profound effect on the actual resistance of any hull to impact.
That said the current theory, which is supported in a number of published studies over the past decade or so, suggests that a cored hull still makes sense if you are designing a hull panel for impact. The operative theory is that while the outer skin of a cored hull may be more likely to be penetrated by a concentrated impact, this is offset in part by the core absorbing a percentage of the impact force, but more significantly the core will distribute the load to a larger area of the interior skin reducing the likelihood that the inner skin will be be breached. The real life reality of this theory, is heavily dependent on the core material, the bonding of the skin to the core, the amount of non-directional laminate in both panel designs.
Coring was also shown to be a factor in the percentage of strength retained over the life of the panel. As you may know, Fiberglass composites have a tendancy to lose a significant amount of strength and ductility over their service life due to flexural fatigue. The greater panel stiffness of a cored panel will generally result in less flexure and so less fatique. Similarly, all things being equal, the greater panel thickness of a cored panel means a lower unit load on the skin and so also less fatique.
Of course, where this theory breaks down is in cases where there has been core rot or the core loses its bond with the skins.